1. Field of the Invention
The present invention relates to a method and apparatus for linearizing the output signal of a voltage controlled oscillator (VCO), and more specifically relates to a linear chirp generator and method for use as a swept local oscillator (SLO) in a wide bandwidth compressive receiver or the like.
2. Description of the Prior Art
A chirp is a frequency modulated signal having a linear frequency change versus time characteristic.
There are several techniques for creating high speed linear chirps. These include impulse excitation of a dispersive delay line (DDL), tuning a nonlinear voltage controlled oscillator (VCO) with a piece-wise predistorted voltage generator, and tuning an ultra-linear VCO with a linear voltage generator.
Present technology for impulse excitation of a DDL can provide time-bandwidth products of approximately 500 to 1000. While this may marginally satisfy the high speed, wide bandwidth requirements of a compressive receiver swept local oscillator (SLO), the resultant chirp suffers from a poor signal to noise ratio (SNR) due to the very high inherent insertion loss of the DDL (approximately 30 dB) as well as the expansion loss (which is also approximately 30 dB).
Tuning a nonlinear VCO with a piece-wise predistorted ramp voltage generator may be used to satisfy the SLO requirements of a compressive receiver. However, the resultant circuit becomes fairly complex and extremely cumbersome to align, as described in the publication, Microwave Receivers with Electronic Warfare Applications, John Wiley and Sons, New York, 1986, authored by J. Tsui. The complexity is a result of the required linearity, which is preferably less than 0.15%. To linearize the nonlinear VCO, the tuning curve is broken down into many small pieces. Each piece must be separately timed and adjusted, and then be recombined to form the final predistorted tuning voltage. This process is usually empirically defined due to the difficulties in measuring the dynamic frequency versus time characteristics of the resultant chirp.
An ultra-linear VCO driven with a linear voltage generator can satisfy the SLO requirements of a wide bandwidth compressive receiver, as described in the article, VCO Based Chirp Generation For Broad Bandwidth Compressive Receiver Applications, 1993 IEEE MTT-S International Microwave Symposium Digest, pp. 1113-1115, authored by J. Levy, P. Burke, L. Cohen, and R. Cecchini. A millimeter wave VCO operating at 50 GHz, for example, with an ultra-linear tuning characteristic, for example, 0.1%, can create the chirp. However, local oscillators, filters and frequency conversion components are required to shift the chirp to the microwave frequency range. This complexity results in increased cost, power consumption and size as compared to creating a chirp directly at microwave frequency. Also, incidental FM (Frequency Modulation), which is usually a function of the center frequency, is greater at the up-converted frequencies than at baseband.